Air Sterilization

The magnitude of the air sterilization problem is seen from
the usual needs of a highly aerobic fermentation
where roughly 1 volume of air per volume of medium
per minute may be used. For a factory with 20 fermenters of
100,000 L each, 2 million L/m ( 70,000 ft3 /m ) of air is handled.
Very large compressorsare used, and at least two are required so that
one can be down for maintenance.

In the past, air filters were columns that approached diameters of
one-fourth of the fermenter diameter. The packing was slag wool that
lumped up with repeated use, fiberglass that broke down because of
repeated thermal expansion and contraction, or beads of carbon that
sometime underwent spontaneous combustion and melted the column.
Carbon packing works fairly well but is too bulky. Currently, there
is a pronounced trend to use of membrane filters in a
cartridge configuration
for air
sterilization to obtain excellent performance with units of
relatively small size. Moisture is bad for all methods of air
sterilization and may help microorganisms to pass. A membrane pore
size of 0.2 to 0.3 micrometers is recommended. Hydrophillic
membranes should not be used because moisture held tightly in the
pores is not dislodged unless there is quite high pressure drop
across the membrane. Moisture tends to drain from hydrophobic
membranes and collect in a sump. The units are modular and housed
in a shell with a manifold. Sizing is based on the number of
cartridges needed.

Air leaving a vessel in which pathogenic organisms are
cultured is sterilized by heating. Air in a room for
culturing microorganisms may be exposed to ultraviolet light
to reduce the number of potential contaminants.
Ultraviolet light penetrates poorly through glass, so organisms
in shake flasks are not killed.
Usually, a single light switch turns on white light before a person
enters, and the u.v. goes on when the person flips the switch on leaving.

There are also ultraviolet lights mounted in flow devices for
water sterilization, but quartz bulbs or enclosures are needed to circumvent
the attenuation of u.v. wavelengths. Such devices
are also plagued by turbidity in the water and by dirt
forming on the transparent surfaces.

There have been some attempts to commercialize enzymatic
sterilization of air. The basic concept is to bring microorganisms
and viruses into contact with enzymes that attack nucleic acids.
Viruses are destroyed by passage through a labyrinth of surfaces coated
with deoxyribonuclease enzymes.